I am come down (katabebēka). Perfect active indicative of katabainō. See note on Joh 6:33 for frequent use of this phrase by Jesus. Here apo is correct rather than ek with tou ouranou.
Not to do (ouch hina poiō). “Not that I keep on doing” (final clause with hina and present active subjunctive of poieō).
But the will (alla to thelēma). Supply hina poiō after alla, “but that I keep on doing.” This is the fulness of joy for Jesus, to do his Father’s will (Joh 4:34; Joh 5:30).